1. Field of the Invention
This invention relates to a current limiting horn device for a transmission line.
2. Description of the Prior Art
Recently, an arrester element, mainly made of zinc oxide crystallized at high temperatures to have an appropriate non-linearity, has been developed and applied to arresters for substations. In order to prevent lightening stroke accident on transmission lines, moreover, an arrester insulator having an arrester element sealed in a porcelain tube or an arrester insulator integrally formed only with an arrester element is also being investigated. Furthermore, a current limiting horn which is a combination of an arrester element and an arcing horn of an insulator for power transmission is being studied.
With the arrester insulator above described, when the arrester element is out of order or a creeping short-circuit occurs along insulators, a charging portion and a steel tower are short-circuited resulting into an earthing accident. In this case, it becomes impossible to effect the power transmission until the arrester insulator is electrically disconnected from the power transmission system. Furthermore, a steady state voltage is being applied to the arrester insulator, so long as the transmission line is operative. In order to maintain the electrical characteristics of the arrester element to an acceptable level in spite of its deterioration due to ageing and ensure the long life of the element, therefore, the arrester element is required to be enough long.
On the other hand, with the current limiting horn above described, even if an earthing accident occurs due to the arrester element being out of order, it is possible again to start the power transmission without disconnecting the arrester element from the power transmission system with particular means. Moreover, as voltage is not normally applied to the arrester element, all that causes its deterioration is only the lightening surge current, so that its long life can be expected even if its length is short.
In order to guard a transmission line against the earthing accident with the aid of current limiting horns, however, it is absolutely necessary to cause flash-over between the current limiting horns when being subjected to lightening stroke so as to avoid flash-over along a set of insulators or between conductor and a steel tower. It is therefore required to suitably select mounting positions (coordinates) of the current limiting horns.
Moreover, it is needed for the current limiting horns to take a particular precaution because the zinc oxide applicable to the current limiting horn has a high resistance prior to its operation against the lightening surge voltage to exhibit characteristics different from those of metallic arcing horns used in the prior art insulators.
In more detail, referring to FIG. 1 illustrating a current limiting horn device for a transmission line, a steel tower 1 has an arm 2 from which are hanged a set of insulators 3 for supporting the transmission line 4. The set of insulators 3 comprise at a charging portion a current applicable side horn 5 and at an earthing portion an earthing side current limiting horn 6 using an arrester element 7 mainly made of zinc oxide. When a lightening stroke voltage is applied to the transmission line 4 to cause flash-over, the flash-over passage can be classified into four cases shown in FIGS. 2a, 2b, 2c and 2d. Among these cases, the flash-over passage as shown in FIG. 2a exhibits a sufficient current limiting effect. In the cases shown in FIGS. 2b-2d, however, the current limiting effect cannot be expected and in earthing accident cannot be avoided.